Meteorology: Understanding the Atmosphere Ackerman and Knox
The outgoing longwave radiation (or the OLR) is the amount of energy emitted to space by Earth. Low values usually indicate cold temperatures while high values are warm areas of the globe.
The minimum in OLR, or the longwave emitted flux near the equator is due to the high cloud tops associated with the inter-tropical convergence zone (ITCZ), a region of persistent thunderstorms. This minimum migrates about the equator as seen in the monthly mean maps, and is also seen as a maximum in albedo. Notice how it is difficult to observe the oceanic stratus regions we observed in the albedo maps. This is because the temperature of the clouds is similar to the surrounding oceans, making it difficult to observe.
Observe how the major deserts have their largest OLR during their summer. This results from the annual temperature cycle -- as the desert surface heats up it emits more longwave radiation. Note also the large emission in the vicinity of the oceanic subtropical highs (30N and 30S).
You can even see the onset of the southwest summer monsoon over the Indian subcontinent in June and July.